This invention concerns a multi-well filter strip useful in the assay of microliter quantities of a fluid.
Two basic analytical procedures utilizing microporous membrane filters are known. The first is simply the use of a microporous membrane to collect small particles for analysis, e.g., the collection of trichloroacetic protein precipatates on cellulose acetate membranes. The second is use of the filter in a more active sense wherein a first substance is immobilized in the filter and a reactant is applied to the filter for reacting with the substance, e.g., the immobilization of single strand RNA or DNA on a nitrocellulose membrane for use in nucleic acid hybridization. See Gillespie, D. and Spielgelman, S. Journal of Molecular Biology, 12: 829-842 (1965).
The scale of these analytical procedures continues to decrease such that a 96-well microplate utilizing test volumes of 100 to 200 microliters per well is now widely used. As the apparatus is miniaturized, it becomes more difficult to prevent cross-contamination of samples in adjacent wells and it becomes important to prevent any loss of the fluid from the small sample in the well during the reaction period. For example, U.S. Pat. No. 4,526,690 to Kiovsky et al. describes a 96-well apparatus utilizing test samples of 100 microliters and less. A complex filter structure is provided consisting of a continuous microporous membrane or ultrafiltration media 26 attached to the bottoms of the 96 wells, and to prevent contamination and dehydration, a hydrophobic fabric 30 is disposed below the membrane with a minute space 34 between the membrane and fabric and a guiding spike 60 is provided below each well. The hydrophobic fabric is said to prevent a loss of fluid by lateral migration and/or gravity flow from within the well in the absence of an applied differential pressure across the membrane. The spike is said to serve the dual functions of preventing loss of a pendant drop from the well during the reaction period and second as a guide by which fluids forming as a pendant drop are directed to the appropriate well chamber below.
Another multi-well test device is described in U.S. Pat. No. 4,407,943 to Cole et al. A molded plastic tray 12 has a detachable cover 18. The cover has a plurality of open-bottom cylindrical recesses 20 each having a membrane 22 heat-sealed to the lower rim of the recess. In use, a sample is poured into a recess and flows through the membrane into the tray. The membrane is washed and treated with reagents as needed to detect the presence or absence of a particular material in the sample (col. 9, line 63 to col. 10, line 13).
Another multi-well test device, described in U S. Pat. No. 4,246,339 to Cole et al., includes telescoping cover and base members 10, 12 defining a liquid reservoir therebetween, a resilient foam member 16 for biasing the members in an open position, a sorbent material 18 in the base member having a surface layer 36 and a bulk portion 38, and a plurality of wells 20 in the cover each of which has a bottom opening covered by a microporous membrane 22 sealed liquid-tight to the circumference of the opening. The sorbent material between the cover and base members is spaced from the membranes in the open position, but in contact therewith in the closed position. Liquids are passed from the wells through the membranes into the sorbent means by depressing the members to the closed position. The membranes may be washed to remove unreacted materials by relaxing the top member, placing wash liquid in the test wells, and again depressing the top member to pass the wash liquid through the membranes into the sorbent material. The upper surface layer of the sorbent material resists wetting by the liquid passing through the wells to minimize rewetting the membranes with previously-used liquids.
It is an object of this invention to provide a multi-well filter strip for use in a variety of filtering and assay procedures which minimizes contamination between adjacent wells and permits testing of very small microliter samples.
Another object is to provide a multi-well filter strip wherein the wells are removably attachable to a holder and to each other to permit selected testing of various wells.
Another object of the invention is to provide a multi-well filter strip wherein the contents of the wells are not contaminated by handling the strip and wherein radioactive or toxic specimens in the wells need not be contacted by the user.
Yet another object is to provide a multi-well filter assembly which can be used for prefiltration prior to the assay utilizing small sample volumes with good recovery.
A further object is to provide a multi-well filter strip for use in a vacuum manifold.
A still further object is to provide a vacuum manifold having means for positioning a filter strip and for directing filtrate which exits the filter strip.
A still further object is to provide a vacuum manifold having a structure for supporting the filter membranes to prevent separation of the membrane from the filter well while a differential pressure is applied across the membrane.
Yet another object is to provide a combination filter strip, transfer plate, and multi-well test plate wherein the transfer plate directs the filtrate passing through the membrane into a corresponding well of the test plate.